Time-Division Duplex (TDD) is the application of time-division multiplexing to separate uplink and downlink signals at the same frequency. Signalling is divided into time slots where some time slots are allocated for uplink and others are allocated for downlink communication It may thus emulate full duplex communication over a half duplex communication link.
Time division duplex is advantageous in the case where there is asymmetry of the uplink (UL) and downlink (DL) data rate transmission requirements or demands. As the amount of uplink data increases, more UL communication capacity can be dynamically allocated, and as the UL traffic load becomes lighter, UL capacity can be released and taken away. The same applies in the downlink direction.
For radio systems that aren't moving quickly, another advantage is that the uplink and downlink radio paths are likely to be very similar. This means that techniques such as beamforming based on radio channel reciprocity work well with TDD systems.
Examples of Time Division Duplexing systems include the following communication standards. UMTS 3G supplementary air interfaces TD-CDMA for indoor mobile telecommunications. The TD-LTE 4-G, TD-SCDMA 3-G mobile communications air interface. DECT wireless telephony. Half-duplex packet mode networks based on carrier sense multiple access, for example 2-wire or hubbed Ethernet, Wireless local area networks and Bluetooth, can be considered as Time Division Duplex systems, albeit not TDMA with fixed frame-lengths. IEEE 802.16 WiMAX and PACTOR.
A potential problem with TDD, since both uplink and downlink signalling occurs at the same frequency, is that radio communications which occur in close vicinity of each other between devices aimed for different communication links interfere with each other unless the communications use the same allocation of uplink and downlink time slots. For example, two mobile devices in the same area may interfere with each other if one transmits and the other tries to receive radio signals from an other communication source at the same time.
EP 1 229 671 discloses time slot allocation in the CDMA-TDD scheme. A time slot of uplink is allocated to a mobile station in a cell. At the same time, a time slot of downlink, having the same time slot number as that of the foregoing time slot, is allocated to another mobile station in an adjacent cell, provided that the two mobile stations are distant from each other.